Efficient under color removal

ABSTRACT

A method of performing color space conversion and under color removal. Pixelated data is first resampled ( 102 ) to an efficient word size—typically 8-bits wide. The RGB color format data words are then subtracted from a maximum intensity to yield CMY color format data words. The minimum value of the three intensity words for each pixel is determined and used as an initial black value. A scaled version of the initial black value is then subtracted from each of the four CMYK intensity words ( 104 ). The resulting CMYK values are then limited to a range bounded by the maximum and minimum producible intensities for each color.

This application is a Divisional of application Ser. No. 09/392,211, filed Sep. 8, 1999 now U.S. Pat. No. 6,646,761 and Provisional Application No. 60/100,499, filed Sep. 15, 1998.

FIELD OF THE INVENTION

This invention relates to the field of printer systems' more particularly to the printing of pixelated computer-generated text and images, still more particularly to the conversion of pixelated RGB-based image data into CMYK-based image data for printing.

BACKGROUND OF THE INVENTION

Printing technology is undergoing a revolution as the inexpensive computational power developed for the desktop computer market is applied to the printing industry. Paralleling this growth are the skyrocketing expectations of consumers. While just a few short years ago the goal was to find an acceptable replacement for typewritten text, now the demand is for photographic quality reproduction of both scanned images and computer-generated graphics. Meeting this demand required a significant increase in the resolution of the printed images—both spatial resolution and color depth—that demanded the application of massive amounts of processing power and memory.

Additional demands are placed on the processing power of a printer system by the need to convert images from the Red-Green-Blue (RGB) format used by virtually all CRT-based computer displays to the Cyan-Magenta-Yellow-Black (CMYK) format used by most print engines.

Furthermore, while image quality continues to increase, consumers continue to demand higher and higher print speeds from today's desktop printers. This expectation also leads to greater demands on the throughput of the print engine's processor. Thus' there is a need in the art for systems and methods to increase the efficiency and throughput of print engines.

SUMMARY OF THE INVENTION

Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides system and method for efficient under color removal. One embodiment of the claimed invention provides a method of performing color space conversion and under color removal comprising the steps of providing an intensity word for a first, second, and third color for each of four pixels, subtracting each intensity word for the first, second, and third colors from a maximum intensity word to obtain an initial intensity word for a first, second, and third complementary color, comparing the first, second, and third complementary color initial intensity words for each pixel to determine a minimum color intensity word for each pixel, setting an initial combined color intensity word equal to the minimum color intensity word for each pixel, determining an output intensity word for each of the three complementary colors each of the output intensity words for said complementary colors equal to the initial complementary color intensity word minus a scaled value of the initial combined color intensity word, each of the output intensity words for the complementary colors limited to a range bounded by a minimum and maximum producible intensity, and determining an output intensity word for the combined color, the output intensity word for the combined color equal to the initial combined color intensity word minus a scaled value of the initial combined color intensity word, the output intensity word for the combined color limited to a range bounded by a minimum and maximum producible intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an efficient method and system for translating pixelated image data from an RGB-based CRT display system to a CMYK-based printer system.

FIG. 2 is a schematic diagram of four parallel processors performing a byte-wise comparison of two 32-bit input words.

FIG. 3 is a schematic representation of one implementation of under color removal that minimizes the processing required to perform the scaling, subtraction, and output limiting functions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to maximize the performance of a print engine, it is critically important to optimize the core functions that are done repetitively for each pixel printed. A typical print engine is capable of printing simple text-based pages or pages combining many components, such as a background color, text having a specified font and location, and bitmapped images. Regardless of the contents of each page, each pixel of the desired image typically is represented by multiple words of image data.

Image data is defined as a sequence of multiple-bit words representing an image. Each word represents the intensity information for one color and one pixel. For example, three image data words are used to represent a single pixel using the RGB color format—one word representing the intensity of the red primary color component, one word representing the intensity of the green primary color component, and one word representing the intensity of the blue primary color component. Likewise, four words are used to describe a single image pixel in the CMYK color format—one for cyan, one for magenta, one for yellow, and the fourth one for black.

Although the description of the disclosed invention will focus on only the RGB and CMYK color formats, it should be understood that other primary colors may be substituted for the disclosed primary colors with appropriate adjustments. The use of the RGB and CMYK primary color formats is for purposes of illustration, not limitation, since the RGB and CMYK color formats are by far the most common tri-stimulus primary colors used to create luminous images and printed documents.

FIG. 1 is a block diagram 100 of an efficient method and system for translating pixelated image data from an RGB-based CRT display system to a CMYK-based printer system. The block diagram of FIG. 1 includes an image data resampling block 102, an RGB to CMYK conversion block 104, and lookup tables 106, 108, 110, 112 for each of the four CMYK component colors.

The optional image data resampling block 102 receives image data an converts the word size of the image data to a word size that the RGB to CMYK conversion block 104 can efficiently process. For example, depending on the image source, image data is comprised of a series of 1, 2, 4, 8, or 12-bit samples. Print engines may require an 8-bit data word. Additionally, video processors such as the TMS320C80 manufactured by Texas Instruments Inc. can process image data very efficiently in 8-bit words. Therefore, to provide commonality between the image processing hardware and the print engine, it is efficient to convert the 1, 2, 4, and 12-bit input words into 8-bit data words.

According to one embodiment of the disclosed invention, the image data resampling block 102 is one or more lookup tables designed to convert a digital input having a first word size into a digital output having another word size. For example, a 2-bit input word having possible values of 00, 01, 10, and 11 is converted to an 8-bit output word corresponding to the magnitude of the input word—namely hexadecimal ^00, ^55, ^AA, or ^FF.

The image data resampling function is possible to perform in software or even using specialized dedicated circuitry. Using the lookup table method described above, however, saves two clock periods per pixel compared to the conventional software implementation—a significant savings considering three image data words must be converted for each pixel of image data.

Once the image data is converted to a word size that the processor may efficiently process, the image data is converted from RGB color format, which is convenient for CRT-based computer displays, to CMYK color format, which is efficient for printer systems. Color conversion is a two-step process. The first step consists of generating an intensity word for each of the three primary colors in the CMYK format. The second step is the generation of a black component and the alteration of the three primary colors to compensate for the added black component and to produce a better approximation of the original color.

Since the RGB and CMY color formats are comprised of complementary primary colors, converting between the two color formats merely requires subtracting the intensity value for each primary color from a maximum intensity value. This relationship is represented by the following equations where each letter represents the intensity of that primary color in the RGB or CMYK color formats: C=1.0−R M=1.0−G Y=1.0−B

This color conversion step is efficiently performed four pixels at a time by using the multiple video processors of a TMS320C80 digital signal processor or equivalent. FIG. 2 is simplified schematic diagram of the multiple video processor (MVP) of the TMS320C80 manufactured by Texas Instruments Incorporated. Each of the arithmetic logic units 202 (ALU) of FIG. 2 is capable of executing the same instruction on a separate byte of data. The ALUs 202 also work together to execute some instructions using 32-bit doublewords (dwords) of data from the input source registers 204, 206. When operated independently, the four ALUs process single color pixel information for four pixels simultaneously. Thus, in one processor clock period, four pixels of red data are converted to four pixels of cyan data.

After the input image data is converted to CMY color data, the print engine determines the amount of black represented by the three primary color data words for each image pixel. Although pixels formed using only the three primary color data words have the correct image color, it is more efficient to use a black component than to rely on an equal combination of the three color components to create black.

Since equal amounts of the three primary colors combine to form black, the magnitude of the black ink component is theoretically equal to the minimum of the three primary color words representing each image pixel. Ideally, an amount equal to the entire black component is subtracted from each of the three primary colors—in effect completely transferring the black component from the CMY data words to a separate K data word. There is not, however, a one-to-one correlation between the minimum primary value and the value subtracted from the primary colors.

The minimum color intensity word, that is the initial black value, is determined for four pixels in only two byte-wise comparisons using the MVP. A dword comprised of intensity data for a first color for four pixels is compared to a dword comprised of intensity data for a second color for the same four pixels. The byte-by-byte minimums from this comparison are then compared to a dword comprised of intensity data for a third color for the same four pixels. The four bytes resulting from the second comparison are the minimum color intensity words for each of the four pixels.

For example, cyan image data for four pixels (C₁, C₂, C₃, C₄) is loaded in a first input register 204 and magenta image data for four pixels (M₁, M₂, M₃, M₄) is loaded in a second input register 206. The two dwords are compared on a byte-by-byte basis by the four ALUs 202 and the minimum of the two values forms a dword output 208. The dword output 208 represents the minimums of the two colors, in this example the cyan data for pixels 1 and 4 and the magenta data for pixels 2 and 3 (C₁, M₂, M₃, C₄). This intermediate value (C₁, M₂, M₃, C₄) is then compared to the yellow image data for the four pixels. The byte-by-byte minimums from this second comparison (C₁, M₂, M₃, Y₄) represent the minimum of the three primary colors for each of the four pixels and are the black (K) image data words for each of the four pixels. In this example, only the yellow data for the fourth pixel is less than the minimums from the first comparison. Therefore, the black image data for the four pixels is C₁, M₂, M₃, and Y₄.

Although shown as separate input and output registers in FIG. 2, the registers are implemented in multiple ways. For example, the input and output registers may be part of a dedicated register file accessible by the ALUs. Alternately, the input and output registers shown in FIG. 2 are locations in a memory array.

In practice, it is advantageous to scale the combined color component, or black component, as well as the amount of the combined color component subtracted from the three primary color words in order to improve image quality. Scaling the combined color component results in part of the black image component being formed by black ink and part formed by a combination of the three primary colors. The scale factor used may range from −1 to +1 times full-scale, or −255 to +255 for an eight-bit number.

The combined color value is typically scaled using a memory lookup table. The scaling, which is sometime referred to as gamma correction, adjusts the combined color value to compensate for the non-linear properties of the printer. The scaling optionally performs a screening operation. For example, the scaling can limit the maximum value of the combined color value in order to perform threshold screening. To make the process more efficient, all of these functions—black generation, non-linear compensation, and screening—are combined into a single lookup table.

The initial black value, or initial combined color intensity word, is equal to the minimum of the three color intensity words for a given pixel, and is used as an index into the memory lookup table. The value stored at the location pointed to by the index is used for the actual black value, or the output intensity word for the combined color. The values stored in the black generation lookup table range from zero to the maximum allowable black level. Therefore, the black generation lookup table performs the function: black=min(1.0,max(0.0,BG(k))) where BG(k) is the combined lookup table for both the black generation and the transfer function, k is the initial black value, and black is the actual black value—the output intensity word for the combined color.

The actual output intensity word for each of the three primary colors, or each of the three complementary colors when color conversion and under color removal are combined, is determined by the following equations: cyan=min(1.0,max(0.0,C−UCR(k))) magenta=min(1.0,max(0.0,M−UCR(k))) yellow=min(1.0,max(0.0,Y−UCR(k))) where UCR( ) is the under color removal transfer function, k is the initial black value—that is the initial combined color intensity word—and C, M, and Y are the initial intensity words for each of the three complementary colors.

As shown above, the initial black value is also used to index an under color removal lookup table to determine how much each color is reduced to compensate for the addition of the black ink. Each color uses the same UCR lookup table. Each color, however, may have a unique transfer function lookup table 106, 108, 110, as shown in FIG. 1.

The UCR lookup tables for the primary colors are typically 2-bytes-wide in order to provide sufficient precision while allowing the scaled values to range from −255 to +255. Since two bytes are used to store the UCR values' but only nine bits are required for the −255 to +255 range, the upper byte will be “0000 0000” for positive numbers and “1111 1111” for all negative numbers.

The scaled value of the black component generated by the UCR lookup table is subtracted from the initial value for each color in order to obtain the output intensity word for each color. The output intensity word for each color is limited to values between 0.0 (no color component contribution) and 1.0 (maximum color component contribution). As discussed above, it is advantageous to minimize the processing required to perform the scaling, subtraction, and output clipping.

One method of minimizing the processing necessary to perform the scaling, subtraction, and output clipping takes advantage of the 2's complement format in which the data in the UCR lookup table is stored, and the multiple ALUs available in modern signal processors such as the MVP. Using the MVP allows under color removal, as well as clipping the data to a maximum value of 1.0 or clamping the data to a minimum value of 0.0, if necessary, to be performed on four pixels of single-color data in only two processor clocks.

As mentioned above, the UCR tables are comprised of two bytes for each possible index value. Since color data is limited to 8-bits (0 to 255), the data in the UCR table is limited to −255 to +255—a range which only requires 9-bits of data. The most significant byte is the sign byte and contains all zeros in the case of a positive number and all ones in the case of a negative number. The sign byte is not used during the actual subtraction of the scaled black value from the initial color intensity word. This subtraction is performed as unsigned subtraction in which the subtrahend is complemented and added to the minuend along with an injected carry into the LSB. Sample code for the MVP is: C_reg=mc C_reg−UCRreg where C_reg contains a byte of intensity data for each of four pixels' and UCRreg contains the magnitude byte of data for four pixels. The “mc” modifier specifies byte-wise operation and sets the carry flags if necessary.

Code for reading the black generation (BG) and under color removal tables (UCR) is listed below:

;*** START OF CODE ******************************** ;**************************************************** ;BG and UCR LUT readings are done here. BG(k) returns ;the output K. UCR (k) can be positive or negative ;UCR is stored in 16 bit 2's complement form. Here the ;sign ( first 8 bits ) and the magnitude (lsb 8 bits) ;of UCR are separated and stored at different locations ;for faster future usage. Please note that previous ;loops have given the minimum value of c, m, & y and is ;store in Ga_out_mink location. Each time one byte ;is loaded for processing here. ;**************************************************** BG_UCR_LOOPSTART: tempreg0 =ub UCR2 >> 8 ∥ Lx_k1 =ub *(Ga_outmink++)  ; load k1 ∥ *(La_outK++)=uh BG2  ;this is the final BG output as BG(k) ;store the final value of K as BG is ;planned to incorporated transfer fn ;also signpacked =uh UCR1 & ~%8 + tempreg0 ∥ Gx_k2 =ub *(Ga_outmink++)  ; load k2 BG1 =ub * (La_BGLUT + Lx_k1)  ; BG LUT1 ∥ *(Ga_outUCRsign++) =uh signpacked UCRpacked =uh UCR1 \\ 8 | UCR2 & %8 ∥ BG2 =ub * (Ga_BGLUT + Gx_k2) ∥ UCR1 =uh *(La_UCRLUT + [Lx_k1]) le0_BG_UCR_LOOPEND: BG2 =uh BG2 + BG1 << 8 ∥ UCR2 =uh * (Ga_UCRLUT + [Gx_k2]) ∥ *(La⁻outUCR++) =uh UCRpacked ;*** END OF CODE *********************************

After the subtraction, the data is clipped and clamped by a logical operation using the intermediate results in the color register, C_reg, the byte-wide sign information from the UCR table, and the carry flag which is expanded to 8-bits for each of the four ALUs. Sample code for the MVP is: C_reg=(C_reg|(UCRsign & @mf)) & (UCRsign|@mf) where UCRsign is a byte of sign information for four pixels of data which is “1111 1111” when the subtrahend is a negative number, and @mf is the carry out flag expanded to eight-bits. The result of this logical operation is that each byte of the C_reg is clamped to “0000 0000” when the subtraction operation should otherwise return a negative number (subtract a positive number and no carry—UCRsign and @mf both equal “0000 0000”), clipped to “1111 1111” when the subtraction operation should otherwise return a positive number greater than 8 bits (UCRsign & @mf both equal “1111 1111”), and otherwise unaltered.

Sample MVP code for the subtraction, clip, and clamp operation is:

;*** START OF CODE ******************************** ;**************************************************** ;The sign and magnitude (even though it is 2's complemented) ;are stored at different locations. Depending on the sign and ;magnitude the numbers are added/subtracted to the UCR loop. ;please follow the M_reg flow which should explain the things in ;the loop. UCRreg is stored with 4 pixels of UCR data where ;as UCRsign contains the sign of these 4 pixels. ;**************************************************** ADD_SUB_LOOPSTART: C_reg =mc C_reg - UCRreg ∥ *La_Yptr++ = Y_reg C_reg = (C_reg | (UCRsign & @mf)) & (UCRsign | @mf) ∥ M_reg = *La_Mptr M_reg =mc M_reg - UCRreg ∥ *La_Cptr++ = C_reg M_reg = (M_reg | (UCRsign & @mf)) & (UCRsign | @mf) ∥ Y_reg = *La_Yptr ∥ UCRreg = *Ga_UCRregptr++ Y_reg =mc Y_reg - UCRreg ∥ *La_Mptr++ = M_reg ∥ UCRsign = *Ga_insignptr++ le0_ADD_SUB_LOOPEND: Y_reg = (Y_reg | (UCRsign & @mf)) & (UCRsign | @mf) ∥ C_reg = *La_Cptr ;*** END OF CODE *********************************

FIG. 3 shows a schematic representation of one implementation of the under color removal described above that minimizes the processing required to perform the scaling, subtraction, and output limiting functions. FIG. 3 only shows the under color removal operation for a single color of a single pixel. As described above, the MVP performs the operation for four pixels at a time.

In FIG. 3, the initial combined color word, or black value K, is used as an index into an under color removal lookup (UCR) table 302. The output of the UCR lookup table 302 for each value of black consists of a sign byte (MSB) 304 and a magnitude byte 306 representing the scaled black value to be subtracted from a given primary color initial value word 308.

Arithmetic logic unit (ALU) 310 subtracts the scaled black value 306 from the primary color initial intensity word 308. The output of the ALU 310 is a byte of data 312 and a single carry-out bit that is internally expanded, or replicated, to a byte 314. The carry-out byte 314 is logically ANDed with the UCR lookup table sign byte 304 by AND gate 316. The output of gate 316, actually eight gates is “1111 1111” when the data byte 312 is greater than 255—that is, when an overflow condition occurred in ALU 310. OR gate 318 replaces the output of the ALU with all ones to clip the output to 255 when there has been overflow.

The output of OR gate 318 is gated out by AND gate 320. When the subtrahend is negative or there is a carry-out condition, the output of gate 318 is gated out—which is the data output 312 from the ALU 310 unless there is both a carry-out and negative subtrahend, in which case all 1's are output. No carry-out with a positive subtrahend indicates a negative result which is clamped at “0000 0000” by AND gate 320.

Thus, although there has been disclosed to this point a particular embodiment for efficient under color removal, it is not intended that such specific references be considered as limitations upon the scope of this invention except insofar as set forth in the following claims. Furthermore, having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, it is intended to cover all such modifications as fall within the scope of the appended claims. 

1. A method of under color removal, the method comprising: providing an initial color value as a minuend; providing an under color value as a subtrahend wherein at least a most significant bit of said subtrahend is a sign bit; and subtracting said subtrahend from said minuend to yield a carry-out signal and an output color result representing said initial color value net of said under color value; limiting said result by: logically ORing each bit of said result with the logical AND of said carry-out and said sign bit; and logically ANDing each bit from said ORing step with the logical OR of said carry-out bit and said sign bit to yield an output bit.
 2. The method of claim 1, said providing an initial color value comprising providing an 8-bit initial color value word.
 3. The method of claim 1, said providing an initial color value comprising providing a word and said providing an under color value comprising providing a double word.
 4. The method of claim 1, said providing an under color value comprising providing a 16-bit word.
 5. The method of claim 1, said providing an under color value comprising providing a 16-bit word in which a most significant 8-bits have the same value.
 6. The method of claim 1, said providing an under color value comprising providing a double word in which all of the bits in the most significant word have the same value.
 7. The method of claim 1, said providing an initial color value comprising providing an initial color value in unsigned binary format.
 8. The method of claim 1, said providing an under color value comprising providing an initial color value in two's complement format.
 9. A circuit comprising: a processing unit receiving a minuend and a subtrahend and outputting a result equal to said minuend minus said subtrahend and a carry-out bit; at least one first AND gate for logically ANDing said carry-out bit and a sign bit of said subtrahend; at least one first OR gate for logically ORing said carry-out bit and a sign bit of said subtrahend; at least one second OR gate for logically ORing each bit of said result with an output of said first AND gate; at least one second AND gate for logically ANDing each output of said at least one second OR gate with an output of said first OR gate.
 10. The circuit of claim 9, wherein said processing unit receives said minuend as an 8-bit binary word.
 11. The circuit of claim 9, wherein said processing unit receives said subtrahend as an 8-bit two's complement word.
 12. The circuit of claim 9, wherein said processing unit receives said subtrahend as a 16-bit two's complement word wherein a most significant 8 bits of said subtrahend are equal to a sign bit. 